Scotland Yard today launched an inquiry after Peter Mandelson, the business secretary, had a cup of green slime thrown in his face by an environmental protester.

A spokesman for the Metropolitan police said detectives would investigate the incident, which took place as Mandelson was arriving at a low-carbon energy summit in London early this morning, but that no one had been arrested.

News cameras showed the minister genially approaching a young woman after he got out of his car and receiving a large portion of the emerald green liquid – later identified as custard containing added colouring – that she threw at him from a few feet. Mandelson ducked, but not in time to prevent the liquid hitting him on the right side of his face, in his eye and on his coat. The young woman then walked away, unarrested.

She identified herself as Leila Deen, 29, a supporter of the Plane Stupid activist group campaigning against airport expansion. Deen told reporters she had been motivated by reports that the minister had held a meeting with lobbyists for the British Airports Authority, which wants a third runway at Heathrow, a few days before the government endorsed the plan.

She said: "The only thing green about Peter Mandelson is the slime coursing through his veins. That he is trying to make political capital out of climate change ... is an insult to my generation. He is unelected and only represents business interests."

Mandelson described the incident as an "adolescent protest". He said: "She was so busy throwing what seemed like green soup or something in my face that she failed to tell me what the protest was about, but, as you can see, thankfully it was not paint and I have come through it intact."

Deen disputed this, saying: "Peter Mandelson is trying to say he doesn't know what it was about, but as I threw it I said, 'This is for the third runway'."

Initially the police indicated that they would only investigate if they received a complaint. But this afternoon a Metropolitan spokesman said: "Police are investigating circumstances surrounding an incident in Carlton House Terrace, SW1, at about 8am today." ."Officers from Westminster CID will investigate. There have been no arrests. No complaint has been received."

Scotland Yard announced its decision to investigate a few hours after John Prescott, the former deputy prime minister, complained that Deen had not been arrested. In a video post on gofourth, a Labour campaign website, Prescott, who famously punched a protester during the 2001 election, after the man threw an egg at him, said politicians should not have to put up with that sort of treatment.

"What is totally unacceptable is the way the woman walked away claiming it was her right in democracy. She should have been arrested. It is not acceptable that she should be allowed to walk away," Prescott said.

Speaking to the Guardian, Deen explained how the protest had come about. "I had heard Peter Mandelson wanted to talk about climate change and a low-carbon future today; that to me was totally galling," she said.

"He's been actively pushing a high-carbon future through the third runway. I didn't want to let him stand up and talk about that, so last night I decided to make some custard, colour it green, and show how slimy I think he is.

"A lot of people will agree with me. Peter Mandelson is overdue for this kind of treatment."

Asked if she thought her action was non-violent, she said: "Yeah, absolutely." It had been "simply harmless custard", she said.

"It is a confrontational action, it is certainly not a violent action. The police came up to me and said that they wanted to check I wasn't going to try to enter parliament. They said they couldn't comment on what I'd done, but they hoped I had a lovely day."

Deen said Mandelson was an anti-green influence on the government. "He came back and suddenly we've got a third runway that nobody wants. It's quite clear whose side he's on. He's from this generation of people who don't care what kind of legacy they're leaving. Our generation don't have that luxury. We have to think about 30 years' time, when the world is facing a catastrophe."

Mandelson said later that he had never been afraid for his safety. "It could have been nastier," he told reporters. "Thankfully it was sort of, I think, organic and non-toxic. Whilst I'm prepared to take my fair share of the green revolution on to my shoulders, I'm less keen on having it on my face.

"I also don't think that anybody should overreact and, if there's a security issue it's for the police and others. I lived with permanent round-the-clock security when I was Northern Ireland secretary. I don't think I need to go back to that. I'm not afraid for my security as long as I'm doing my job, doing it conscientiously, addressing the right issues as we have been this morning.

"That's what I'm paid to do, that's what I'm accountable to parliament for and that's what I'm going to keep on doing."

He added: "In a sense I guess I should be grateful to the protester for helping us to put this very important subject on the map."

Asked what he thought of the reasons behind the protest, Mandelson said: "I would rather people said it to my face than threw it in my face."

Gordon Brown, addressing the summit, also laughed off the incident, saying: "If anybody doubted the greening of Peter Mandelson and his willingness to take the green agenda on his shoulders we've seen it in practice on our television screens already this morning."

Deen's mother, Sheila, said that her daughter had been aware of green issues since a child and that she was a member of Greenpeace herself. At home in Brighton, she said: "I'm proud that she's got the courage of her convictions and she's prepared to take direct action for injustice. It's not easy to take direct action, to know you run the risk of being arrested."

Plane Stupid has held a number of high-profile disruptive events, protesting against airport expansion plans across the country in recent months, most recently earlier this week at Aberdeen airport when a group blocked a helicopter launch pad for a brief period.